the tool allows placing rectangular boundaries; might it be an idea to default to octagonal areas - rectangles with some % of the corners clipped off, to better approximate random shapes in images; then offer a UI option to tweak that shape (change to a rectangle, or adjust),
one problem is objects aligned diagonally, rectangles poorly approximate these. without being too fiddly, a couple of octagonal areas could better approximate those. Or you could have a easy few options, without having to go as far as a 'vertex/edge manipulation'/'general polygonal outlines' e.g.
/===\ default outline shape - octagonal | | \===/
+===+ rectangular | | +===+
/=+ left-diagonal / / +=/
+=\ right-diagonal \ \ \=+
even without adjusting the shape, placing a couple of octagonal regions could better approximate a diagonal region
/===\ /==|\ | | || | | \|==/ \===/